State of the art, wide-bandwidth antennas have resisted dramatic size reductions due to the inability to develop nanostructured materials at a technologically relevant scale. Dual polarized thin printed phased arrays having bandwidths greater than 10:1, where the bandwidth is defined as the ratio of the upper to lower operating frequencies, provide future differentiators enabling superior performance for many applications. An important component in achieving high performance over a wide bandwidth is the development of high impedance nanostructured substrates. These substrates, utilized in printed phased array designs, will support dual polarization, a low profile conformal footprint and a high frequency response from 0.1-5 GHz.
Developing ultra-wide bandwidth conformal apertures requires properties from the inter-cavity substrate unavailable in traditional bulk magnetic materials. These properties include a high resistivity (>10 Ohm-m), a high relative permeability to permittivity ratio (>3), and a greater than unity and a greater than unity relative μ′ and μ″ across the frequency range of interest. The physical properties of the substrate material are governed in a large part by the microstructure of the film. The product of permeability and cut-off frequency is invariant for a material and is the Snoek limit. However, an enhancement of properties occurs when the grain size of the ferrite film is less than about 30 nm. By reducing the dimensionality of the film, permeability is extended to frequencies higher than observed in the bulk solid. As a result, the Snoek limit can be exceeded in these reduced dimensionality materials.
Inverse spinels are amongst the most widely studied materials for magnetic applications due to their favorable magnetic properties. In this structure, magnetization arises only from the magnetic moments of the trivalent M ions due to the antiferromagnetic coupling of the divalent iron ions. The most commonly studied of these materials include ferrous ferrite, nickel ferrite, manganese ferrite, and cobalt ferrite. With the exception of cobalt ferrite, these materials all exhibit soft magnetic behavior, which means that they magnetize and demagnetize easily. Further, these materials display high permeability, saturation magnetization, and electrical resistivity, which make them ideal for high-frequency applications.
The primary challenge preventing the implementation of inverse spinel ferrites into antennae applications is the inability to obtain the necessary electromagnetic properties in films that are sufficiently large in-plane and sufficiently thick. For example, Ni—Zn—Co ferrite thin films have been synthesized by a spin-spray technique with high μ″ at high frequency. These films showed columnar grain structures with grains that were through-thickness (˜1 μm) in height and sub-μm in-plane grain sizes. However, the spin spray technique is not scalable to the very large area substrates and thick films required. This is due to the non-uniformity inherent in the nozzle spray process as well as the inefficient use of precursor chemicals.
There remains a need for improved methods of making ferrite thick films.